The present invention relates to a method of and apparatus for the continuous casting of a metal. More particularly this invention concerns the electromagnetic mixing of a metal as it is continuously cast.
In a continuous casting operation such as described in the above-mentioned copending applications as well as in French patent applications Ser. Nos. 72/20544 now French Pat. No. 2,187,465 and 76/15178 filed 5, 19, 1976, and U.S. Pat. No. 3,941,183, all of whose teachings are also herewith incorporated by reference, a metal is poured in molten condition into the top of a vertically tubular mold. This mold is cooled so that at least the outer portion of the descending column of a metal in the mold is hardened. Thus a hard casting exits continuously from the bottom of the mold. In such arrangements it has been found extremely advantageous to electromagnetically mix the molten metal, normally steel, in the mold.
Such mixing is achieved by forming a relatively powerful magnetic field and causing it to travel vertically along the mold countercurrent to the descending metal. Since the molten metal is normally introduced into the top of the mold by a so-called dip tube at the center of the mold, the molten metal in the mold forms an inverting toroid that descends in the center of the mold and rises along the periphery. Such mixing is extremely advantageous in that it not only speeds the hardening of the metal by increasing the circulation and heat dissipation in the mass and between the mass and the mold, but it also brings impurities that would form inclusions to the surface of the mold where many oxidize and are lost, or where they even can be caught in a thin slag-like layer on top of the column of descending molten metal in the mold. Such electromagnetic mixing eliminates the necessity of flame scarfing which can waste as much as 4% of the casting.
Nonetheless the known methods, although they do considerably reduce the overall quantity of inclusions and do tend to distribute these non-metallic inclusions more toward the center of the casting, do not give reliably even results. Furthermore the known methods do not allow for an adjustment of the subsurface depth of the non-metallic inclusions.